World Trade Organization chief Roberto Azevedo warned Monday against a damaging "escalation" of trade tensions between the United States and other major economies, following a divisive G7 summit.

Trans-Atlantic and other trade wars would have "a long-term impact on the global economy," he said at a Berlin meeting with German Chancellor Angela Merkel and leaders of the IMF, World Bank and other international institutions.

"We must stop this escalation of tensions, the tit-for-tat process is not going to be helpful... new sectors will be brought into the picture, which is something that will harm everyone," he said.

The EU has vowed to retaliate against US metal tariffs imposed by US President Donald Trump, and tensions worsened at a Group of Seven summit in Canada after Trump rejected an attempt to write a consensus statement and bitterly insulted the Canadian host.

The joint communique which Trump rejected had vowed that G7 members would reform multilateral oversight of commerce through the WTO and seek to cut tariffs.

Azevedo warned that "the rising trade tensions risk a major economic impact, undermining the strongest sustainable period of trade growth since the end of the financial crisis.

"But, more than that, the systemic effects could be very damaging, risking a long-term impact on the global economy."

The US president came under fire at the contentious G7 summit of leading economic powers for his "America First" protectionist drive.

Trump lashed out again at US allies Monday in Twitter messages focusing on "massive trade surpluses" that he said the United States has endured "for decades" with its closest allies.